What happens to change Johnny’s position in the family? What role does Dove play in this event? How does Johnny react to his new status?

Jealous of Johnny, Dove is the one who causes his accident by handing him a cracked crucible, well aware that the crack will not be able to stand the heat. His assumption was correct, and the crucible crumbled, and hot, molten silver ran down his hand. Thus, he rid himself of his competition.

"it (will) just about serve Johnny Tremain right - after the insufferable way he (has) been bossing everybody - if the crucible (gives) way and the hot silver...spill(s) all over the top of the furnace", making Johnny "look like a fool". (Dove)